Abstract
Objectives
The efficacy of Gram-stain microscopy for diagnosis of septic arthritis is fundamentally limited by an inherent false-negative rate of 25–50%. The aim of this study was to calculate the sensitivity of Gram-stain microscopy of synovial fluid collected in heparinised containers and to investigate if this collection method improves diagnostic value.
Methods
12-year retrospective study of patients undergoing synovial fluid analysis between 1998 and 2010. Gram-stain result was correlated with culture result and clinical diagnosis. The formula sensitivity = number of true positives/(number of true positives + number of false negatives) was used for analysis.
Results
602 cases of culture proven septic arthritis analysed with Gram-stain microscopy were identified over this time period. All samples were collected in heparinised containers. The most common joint affected was the knee in 390 cases. 568 cases were correctly identified by Gram-stain microscopy as positive and 34 were falsely identified as negative giving a false-negative rate of 5.6%, and a sensitivity value of 94%.
Conclusion
This is the largest study investigating the efficacy of Gram-stain microscopy in the literature. We report a sensitivity of 94% for Gram-stain microscopy, far higher than previously reported (50–75%). No direct comparison was made to non-heparinised synovial fluid samples however we hypothesise that anticoagulation of our samples is directly responsible for this increased sensitivity. Based on these findings Gram-stain microscopy of anticoagulated synovial fluid samples is still a valuable investigation for suspected septic arthritis.
